The semiconductor integrated circuit (IC) industry is arguably the most fast-growing industry in the world during the last several decades. Throughout the world, an incalculable number of semiconductor devices or circuits built on IC chips are needed each year. Thus, the whole IC industry consumes annually a tremendous amount of semiconductor wafers in making these chip products. In addition to the wafers used for making end IC chip products, a significant amount of monitoring wafers, ordinarily 10-15% of total production wafers per IC manufacturing factory, are used for a variety of testings during IC manufacturing processes. For example, for an IC fabricating factory with a capacity of processing 30,000 wafers per month, approximately 3,000 test wafers per month are needed for testing purposes.
The wafer testings during a manufacturing process are essential for the IC chip fabrication. The semiconductor manufacturing processes are extremely sensitive to any variations of operating conditions, such as, operating temperature, pressure, or humidity in the reactor, and are also equally sensitive to variations of manufacturing equipment calibration. For instance, before a batch of wafers may be processed, some test wafers have to be used to detect the particle contamination level of the equipment to avoid the waste of the whole batch. If the particle contamination level is too high, cleaning has to be performed on the equipment prior to any production processes. Also, the operating conditions of the semiconductor manufacturing equipment are extremely delicate. Thus, the testings qualify the stability of manufacturing equipment and allow an operator to make any necessary adjustments to the equipment before they are used to process the wafers. Depending on applications, each semiconductor factory typically runs one or more test per reactor per day for the production processes. As a result, a vast amount of test wafers are used in the IC industry annually.
In the past, few IC manufacturers reclaimed used or test wafers for later uses, and these used or test wafers were normally thrown away. Many factors had contributed to the reasons why previously IC makers did not reclaim the test wafers after being used. For example, the IC makers generally made good profits in the past, the cost of a new wafer was substantially similar to that of a used one, or the qualities of reclaimed wafers might be questionable due to lack of good reclaiming techniques. For whatever reasons, the whole semiconductor industry simply did not endeavor to reclaim used or test wafers. Nevertheless, the situation has notably been changed in the modern IC industry. Nowadays, the profit margin of IC manufacturing is generally declining and the cost savings for using reclaimed wafers have been greatly increased as the wafer size moves from 6 inches to 8 inches and then to 12 inches. Each 12-inch wafer typically produces up to 4 times the number of end chip products than a 6-inch wafer does. Thus, wasting a 12-inch wafer means a potential loss of up to four-times the revenue per wafer of a 6-inch wafer. The semiconductor industry thus began to reclaim test or used wafers during the last several years.
Some conventional techniques to reclaim used or test wafers have been developed during the last several years. In general, these conventional techniques include a first step of acid stripping, a second step of lapping/polishing, and a final step of cleaning the wafers. The acid stripping step is used to remove oxides or metals deposited on the test wafers. A test wafer is usually an unpatterned wafer deposited with oxides or metals, or both, over the test wafer. As a result, the first conventional step in reclaiming test wafers is to strip the oxides and/or metals deposited over the wafers. Stripping the oxides and/or metals is conventionally accomplished by using an acidic base chemical solution to etch and remove the oxides or metals from the wafers. To strip oxides or metals, the wafers may be immersed within a liquid tank containing the acidic chemical solution or the chemical solution may be dripped on the wafers to etch oxides or metals.
Merely using an acid bath to strip the oxides and/or metals is, however, not enough to restore the test or used wafers to their original pre-used condition because the after-stripped wafers normally still contain significant amount of defectivities, which may be particles on the surface or embedded defects in the outermost 1-2 .mu.m depth of the silicon wafers. A restored wafer has to be extremely clean and free of defectivities before it can be reused. For example, during an IC manufacturing process, an important qualifying test for equipment used in the production is a defectivity level test, e.g., to test how many particles have been deposited or how many defects have been formed on the test wafers. As a result, all test wafers have to be virtually defectivity-free before they can be used for any test purposes to assure a reliable test result. Otherwise, if a defectivity-infested wafer is tested, it is impossible to qualify or disqualify an equipment tested due to a high signal/noise ratio. Almost all other applications, such as growing an epitaxial layer over a wafer, also require particle contamination free wafers. Nonetheless, although the acid bath stripping may remove the oxides and/or metals of the wafers, the wafers merely treated by the acid bath are often still infested with unwanted defectivities and are thus unfit for future uses in IC manufacturing. The acid bath treated wafers will thus need further treatments before they can be reused either for testing or for production purposes.
To address the above-mentioned problem of undesirable defectivity level, conventional IC manufacturers use a lapping/polishing technique after the acid stripping treatment to remove particles and/or to polish the wafer for removing surface defects on the wafer. The conventional lapping/polishing technique is essentially a mechanical polishing process using abrasives, normally contained in a chemical solution called slurry, during polishing of the wafer. During lapping/polishing, a polishing pad is used to grind the wafer. Therefore, the lapping/polishing technique may also be used to planarize the wafer, or to remove a layer, such as materials which are not removed by the prior acid bath treatment, on the wafer. During the lapping and mechanical polishing step, deionized water may flow through the wafer to cool down heat generated by polishing and to remove particles and/or scraps during polishing.
The lapping/polishing technique is, however, not without disadvantages. For instance, the abrasives used in lapping/polishing technique are often quite rough. Thus, the wafer polished by lapping/polishing would normally lose at least 25 .mu.m in thickness per lapping/polishing run. Moreover, the lapping/polishing technique is very difficult to be controlled precisely, particularly to a large surface area. As a result, uneven thickness on various portions of a wafer might often result from the lapping/polishing process. Therefore, the lapping/polishing technique is unfit for a larger surface size wafer, such as an 8-inch or a 12-inch wafer, and is often unacceptable to the modern deep sub-micron technology due to its roughness.
The third conventional step in reclaiming a wafer is to clean the stripped-and-polished wafer. Clean wafers are essential at all stages of the fabrication process but are especially necessary before any of the operations performed at high temperature. The cleaning may be accomplished by a conventional non-contact cleaning procedure, i.e., the wafer is not touched by a cleaning equipment such as a brush during cleaning, and the cleaning is usually performed in a wet bench. In the past, the non-contact cleaning is predominantly used to clean the wafer during the manufacturing processes by the semiconductor industry. For instance, a sonic wave cleaning technique is probably one of the most commonly used non-contact cleaning techniques. Sonic waves are energy waves generated by a transducer. Two ranges of sonic waves are often used. The first range of approximately 20,000-50,000 Hz is called the ultrasonic waves and the second range of approximately 850 kHz is called the megasonic waves. In the megasonic/ultrasonic cleaning procedure, the wafer may be immersed in the deionized water or the deionized water may be dripping on the wafer. The sonic waves pass through the deionized water and cause particles or residues on the surface of the wafer to fall off.
The megasonic/ultrasonic cleaning is, however, effective only to remove those particles loosely attached to the wafer. It is not effective when a certain sort of attractive forces, whether chemically or physically, exist between the particles and the wafer. For instance, a particle and the wafer will be attractive to each other if they respectively have opposite electrical charges existing on them. They will also be attractive to each other when a certain kind of chemical bonding force exists between the particle and the wafer. For these situations, megasonic/ultrasonic cleaning is ineffective and other cleaning step is needed to remove the particles or residues on the wafer more effectively in order to provide an extremely clean wafer suitable for reuse.